This invention is directed generally to control of indoor climate modifying apparatus such as an air conditioning unit or a furnace for maintaining comfort for the occupants of enclosures. The more important application is in controlling operation of air conditioners, and the discussion and disclosure following will be based primarily on the air conditioning case. The invention will typically be implemented in an electronic thermostat which uses a microcontroller in conjunction with a temperature sensor for controlling opening and closing of a solid state switch which controls the flow of operating current to the air conditioning control module.
Thermostats typically in use now which direct operation of air conditioners use dry-bulb temperature as the controlled variable. By the term "dry-bulb temperature" is meant the actual temperature of the air as measured by a typical thermometer, and the use of the term "temperature" or "air temperature" hereafter will refer to dry-bulb temperature unless the context clearly directs otherwise. It is easy to measure air temperature and this measurement is already available in most thermostats. A typical thermostat in air conditioning mode causes the air conditioning to begin operating when temperature rises above a set point value. The air conditioner responds by injecting cold air into the enclosure until the temperature within the enclosure has fallen to a point below the set point value. The typical thermostat uses an anticipation element so as to turn off the air conditioning before the actual set point is reached. For many situations this type of control results in air which is comfortable for the enclosure's occupants.
It is well known that an air conditioner removes humidity from the air as well as cools it. The mechanism by which humidity is removed involves passing air from the enclosure or from the outside through the air conditioner, reducing the temperature of this air to substantially less than the comfort range of 70.degree.-74.degree. F. In order to remove humidity from the air, the temperature of at least some of it must be lowered to less than the current dew point temperature, the temperature at which water condenses from the air. Some of the water in the conditioned air condenses on the cooling coils of the air conditioner in this process and drips off the coils to a pan below, from which it drains. Because air will not release any of its humidity until it has reached 100% relative humidity, i.e., its dew point temperature, it is necessary for at least the air adjacent the cooled surfaces of the heat exchanger to reach this temperature. The total air stream through the air conditioner however, may not reach 100% relative humidity because not all of the air is cooled to its dew point. The relatively cold and dry conditioned air (relatively dry even though it has nearly 100% relative humidity) is mixed with the uncomfortably warm and humid air within the enclosure to achieve a more acceptable 40-60% relative humidity at a comfortable temperature of 70.degree.-75.degree. F. as well controlled by the thermostat.
Normally this procedure results in air within the enclosure whose humidity is within the comfort range. However, there are situations that can result in air having humidity which is still too high when the temperature requirement has been met. To achieve air at comfortable levels of both temperature and humidity, an air conditioner is sized for the expected load which the enclosure will present so that when the set point temperature is reached, humidity is acceptable. But in cases of unusually high humidity or where the air conditioner capacity relative to the current environmental conditions does not result in sufficient dehumidification when the set point temperature is reached, it is possible for the air in the enclosure to have excessive humidity.
It seems to be a simple solution to control the relative humidity in the enclosure by simply adding a relative humidity sensor to the thermostat, and then controlling the air conditioner to hold relative humidity within a selected set point range. A problem with this approach is that the relative humidity of the enclosure air may actually rise as the air is cooled and dehumidified within the enclosure. This possibility arises because the relative humidity is a function of both the amount of water vapor in a given volume or mass of air and its dry-bulb temperature. Relative humidity for any volume of air is defined as the ratio of the partial pressure of the water vapor in the air to the vapor pressure of saturated steam at that temperature. Since the vapor pressure of saturated steam drops rapidly with temperature, a relatively small amount of water vapor in a volume of air at a lower temperature can result in 100% relative humidity. It is thus possible to have a runaway situation where the humidity control function in the thermostat continues to call for further dehumidification, and as the temperature within the enclosure falls, relative humidity rises and locks the air conditioning on.
U.S. Pat. No. 4,105,063 (Bergt) is related art which discloses an air conditioning system which controls the dew-point temperature of enclosure air independently of the dry-bulb temperature. Bergt provides a sensor responsive to absolute moisture content which operates in parallel with the normal dry-bulb temperature control. Because of the parallel operation of the two control functions, undesirably short cycles are possible. This over-cycling problem is solved by the present invention. Bergt's invention may also require reheat which reduces the efficiency of the process.
U.S. Pat. No. 4,889,280 (Grald and MacArthur) is related art disclosing an auctioneering controller wherein the predetermined dry-bulb temperature set point is modified in response to a absolute humidity error signal. The enclosure temperature which results may not always be comfortable, and there is also a potential for over-cycling.